1. Field of the Invention
The invention relates to a method of producing dry sodium dichloroisocyanurate dihydrate.
2. Description of the Prior Art
Alkali metal dichloroisocyanurates, especially the sodium salts, are widely known as a source of available chlorine in sanitizing and bleaching applications. These salts are known to exist in the anhydrous form and in the mono and dihydrate forms when associated with bound water. One of the major drawbacks of employing alkali metal dichloroisocyanurates is the property known as self propagating decomposition. It is known that when these compounds are exposed to high temperature source such as a spark or cigarette they can begin to burn and will continue until all of the material is consumed. It is known, however, that the presence of bound water of hydration lessens the tendency of alkali metal dichloroisocyanurates to decompose in this manner. The dihydrate form of these salts is the most desirable in this respect.
One reason why the dihydrate salt of sodium dichloroisocyanurate has not found wide commercial acceptance is the difficulty in manufacturing this product and more particularly problems associated with the drying step. The most common methods described for drying these salts are vacuum drying, drying in an air circulated oven, and spray drying. As noted in Wenzke, U.S. Pat. No. 3,289,312, none of these methods have provided any commercially acceptable product. Wenzke describes a process in which wet isocyanurate salt solids are fed to a pug mill to which dry anhydrous salt is added to reduce the free water content, followed by drying in a special chamber containing a cage-type mill through which a heated gas stream is flowing. This process is designed to produce an essentially anhydrous product.
Among other techniques proposed for drying this type of compound are the spray granulating process disclosed in Saeman, U.S. Pat. No. 4,005,087; the flash drying method described in Nelson, U.S. Pat. No. 3,951,972, the two-stage fluidized bed drying process described in Japanese Patent Publication No. 43-29,588, and the method of Goelz, U.S. Pat. No. 3,818,002, in which an aqueous solution or suspension of the salt is sprayed into a fluidized bed of dry salt particles and dried by the fluidizing gas. Goelz goes to great lengths to disparage both spray drying and fluidized bed drying of dichloroisocyanurate salts.
The alkali metal dichloroisocyanurate salts dried by the prior art methods have not proved commercially acceptable due to poor stability, and excessive caking and dusting.
Thus, it would be highly desirable to provide a method of drying sodium dichloroisocyanurate dihydrate and related salts which could produce a stable, free-flowing, non-dusting powder product.